The present invention relates to an apparatus for generating a strong magnetic field by utilizing an oxide superconductor. More particularly, the present invention relates to a new oxide superconducting wire which is suitable for those apparatuses which need a strong, uniform magnetic field, such as scientific instruments, NMR analyzers, and medical MRI apparatus. It relates also to a solenoid coil and a magnetic field generating apparatus with said oxide superconducting wire and a process for producing said oxide superconducting wire.
Superconducting magnetic apparatuses generally find use in two fields: one in which no consideration need be given to the uniformity and stability of the magnetic field so long as a magnetic field is generated, and the other in which the quality of the magnetic field (or the uniformity and stability of the magnetic field) is a matter of important concern. A highly uniform, highly stable magnetic field is usually required for research work, such as measurements of physical properties and magnetic field generating apparatuses for medical use. It is essential for the technology of high-quality superconducting magnets. Such superconducting magnets have conventionally been produced with accurately worked wires made of a metal superconductor, such as a niobium-titanium superconductor and a niobium-tin superconductor. Superconducting wires are wound into a solenoid under stringent quality control. The resulting superconducting magnet is run with an extremely stable power supply or in a permanent current mode. The thus generated magnetic field is highly stable time-wise and space-wise. Recent years have seen a remarkable advance in magnets generating a highly uniform magnetic field. Magnets capable of generating as high an intensity as 20T have appeared to meet the need for a NMR apparatus. Unfortunately, the intensity of the magnetic field that can be generated by the conventional metal superconductor is limited to about 20T on account of the critical magnetic field inherent in the material used. In order to exceed this limit, it is essential to resort to an oxide superconductor.
A magnetic field generating apparatus has been developed with an oxide superconductor in which the coil is formed by winding a tape-like wire into a double pancake shape, as reported in Journal of Applied Physics, Vol. 35, 1996, Part 2, L623 to 626. This coil is suitable for generating a magnetic field stronger than 22T, which has never been attained with conventional metal superconductors. However, this coil suffers the disadvantage of being unable to generate a uniform magnetic field. The reason for this it that the conductor is in the form of a tape, and the tape is wound into a coil of pancake shape in such a way that more than one coil is placed on top of the other. One way to address this problem is to use a conductor with a round cross section and wind it into a solenoid coil for the magnet, as reported in Journal of Material Science, Vo. 30, 1995, pp. 3200 to 3206. Such a conductor generates a comparatively strong, uniform magnetic field; however, it cannot generate a desired magnetic field because its critical current density (Jc) is only one-fifth that of the tape-like wire. It is possible to secure a strong magnetic field easily by winding a tape-like wire into a solenoid, but it is difficult to secure a sufficiently uniform magnetic field in this way on account of the outer dimensional accuracy of tape-like wire.
A wire with a round cross section is usually produced by drawing a metal through a die, and it has a higher accuracy (of the order of microns) compared with a tape-like wire. Therefore, such a wire is suitable for the generation of uniform magnetic fields. By contrast, a tape-like wire is produced by rolling, and its working accuracy (thickness and width) is usually limited to the order of 10 microns. Therefore, when it is wound in thousands of turns, the number of turns varies from one place to another because of the uneven working accuracy. This results in an uneven magnetic field. For this reason, there has been a demand for an oxide superconducting wire which meets requirements for both high current density and mechanical working accuracy so that it generates a strong, uniform magnetic field.
It is an object of the present invention to provide an oxide superconducting wire which has high dimensional accuracy and generates a strong, uniform magnetic field. It is another object of the present invention to provide a solenoid coil formed from the oxide superconducting wire. The term xe2x80x9cuniform magnetic fieldxe2x80x9d means that the intensity of the magnetic field varies less than 0.1%, preferably less than 0.01%, and the term xe2x80x9cstrong magnetic fieldxe2x80x9d means that the intensity of the magnetic field is higher than 22T. In the past, it was difficult to generate such a strong, uniform magnetic field; it was only possible to achieve it by means of a magnet formed from an oxide superconducting wire.
The basic reason for this is that an oxide superconductor is a greatly anisotropic substance. When it is made into a tape-like wire by rolling, the resulting wire permits a coil current density of about 50 to 100 A/mm2 in a magnetic field stronger than 10T. However, when it is made into a round wire by drawing, the resulting wire permits a current density of only 10 to 20 A/mm2.
An object of the present invention is to provide an oxide superconducting wire which has high dimensional accuracy and generates a strong, uniform magnetic field with a high critical current density. Another object of the present invention is to provide a solenoid coil and a magnetic field generating apparatus formed from the oxide superconducting wire.
According to the present invention, it is possible to produce an oxide superconducting wire which has high dimensional accuracy without a decrease in current density. The first aspect of the present invention is directed to an oxide superconducting wire which is characterized in that the wire has an approximately round cross section perpendicular to its lengthwise direction, the cross section is composed of several units, each unit being composed of a plurality of tape-like oxide superconductors, the tapes in each unit being laminated stepwise on top of the other in the direction perpendicular to the lengthwise direction at an angle of about 60 degrees with respect to the tape surface within the cross section, each unit having an approximately rhombic shape within the cross section, the cross section having at least three different units which are arranged such that adjacent units have a rotational symmetry through about 120 degrees with respect to the direction of tape lamination and at least one side of the rhombic shape of a unit is in contact with an adjacent unit.
In other words, the oxide superconducting wire of the present invention is characterized in that it has a round cross section perpendicular to its lengthwise direction and the cross section is composed of oxide cores which are geometrically arranged at rotationally symmetric positions. This rotational symmetry may be established by 3 rotations, 4 rotations, or 6 rotations within the round cross section. For the oxide to be packed most closely, it is desirable to arrange the cores in triangular symmetry. It follows, therefore, that the most efficient packing ratio (or the ratio of the sectional area of the oxide to the total sectional area) is achieved when three rhombi are arranged in rotational symmetry, each rhombus being composed of two regular triangles.
According to the present invention, the oxide superconductor has a tape-like shape, and these tapes are laminated on top of the other to form a conductor. However, it is very difficult to obtain a conductor of ideal configuration because of the limitation accuracy in tape rolling and tape assembling. In view of this, the angle of lamination direction should be about 60 degrees or 120 degrees. The closer to ideal the shape is, the better the performance will be. As the shape departs from the ideal, the performance of the oxide decreases. The allowance of the angle is about 5 degrees. If the allowance exceeds this limit, the performance will decrease to xc2xd to ⅓.
The second aspect of the present invention is directed to an oxide superconducting wire with a round cross section which, in its cross section perpendicular to its Lengthwise direction, is composed of three units, each unit consisting of laminated tape-like oxide superconductors, the assembly of the units being concentrated to form a closest-packed shape at the center of the cross section, with all the tapes having at least one end thereof in contact with the sheathing material constituting the periphery of the wire.
The above-mentioned rotational symmetry will be satisfactory so long as it is formed within three adjacent units; thus, it is not necessary that all the units form the rotational symmetry in the cross section. Symmetry is essential to prevent the shape from being disturbed during drawing for isotropic reduction in cross section symmetry with three units is not a must; but it is easiest to form. The contact with the sheathing material is not a direct concern of the present invention; it is mentioned here merely from a geometrical point of view.
According to the present invention, the tape-like oxide superconductors may be formed from multi-core wires.
According to the present invention, the tape-like oxide superconductors may be multi-core wires which are twisted.
According to the present invention, the tape-like oxide superconductors may be multi-core wires which are twisted and the tapes are arranged with a high-resistance layer interposed between them.
According to the present invention, the tape-like oxide superconductors may be multi-core wires which are twisted and the tapes are arranged with a high-resistance layer interposed between them, and the superconducting wire is twisted.
According to the present invention, the oxide superconducting wire mentioned above is characterized in that the oxide superconductor should preferably be Bi2Sr2Ca1Cu2Ox. This oxide superconductor may be replaced by others such as (Bi,Pb)2Sr2Ca2Cu3Ox and a thallium-based superconductor.
Examples of the oxide superconductors that can be used in the present invention include:
Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O type:
Bi1.5-2.2xe2x80x94Sr5-2.2xe2x80x94Cu0.5-1.3xe2x80x94O5-7,
Bi1.5-2.2xe2x80x94Sr1.5-2.2xe2x80x94Ca0.5-1.3xe2x80x94Cu1.5-2.3xe2x80x94O7-9,
Bi1.5-2.2xe2x80x94Sr1.5-2.3xe2x80x94Ca1.5-2.3xe2x80x94Cu2.5-3.3O9-11,
Bixe2x80x94Pbxe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O type:
(Biyxe2x80x94Pb1-y)1.5-2.2xe2x80x94Sr1.5-2.2xe2x80x94Cu0.5-1.3xe2x80x94O5-7,
(Biyxe2x80x94Pb1-y)1.5-1.2xe2x80x94Sr1.5-2.2xe2x80x94Ca0.5-1.3xe2x80x94Cu1.5-2.3xe2x80x94O7-9,
(Biyxe2x80x94Pb1-y)1.5-2.2xe2x80x94Sr1.5-2.3xe2x80x94Ca1.5-2.3xe2x80x94Cu2.5-3.3xe2x80x94O9-11,
where y=0-1 to 0.9.
The third aspect of the present invention is directed to an oxide superconducting wire characterized in that the cross section of the wire is round and is composed of several units, each consisting of tape-like superconductors laminated in an approximately rhombic shape, which are arranged such that they form a hexagon as a whole. In other words, the oxide superconducting wire has a round cross section perpendicular to its lengthwise direction within the plane of the cross section, the oxide superconductor is composed of several units, each consisting of a plurality of tape-like superconductors laminated one over another. The laminated tapes form an approximately rhombic shape. Within the plane of the cross section, there are at least three units, which are rotationally symmetric with adjacent units in the direction of tape lamination. At least one side of the rhombus is opposite to the adjacent unit.
The fourth aspect of the present invention is directed to an oxide superconducting wire composed of a metal sheath and a core as an assembly of oxide superconducting filaments, characterized in that the core is made up of multi-core tape wires, each consisting of oxide superconducting filaments, which are arranged in rotational symmetry, the oxide superconducting filaments having a cross section such that the average thickness is 3 to 20 xcexcm and the average aspect ratio is larger than 2 and smaller than 10. The oxide superconducting tape-like wires are arranged in rotational symmetry. This step is accomplished when the multi-core tape-like wires are packed in the third metal pipe which becomes the metal sheath later.
The fact that the multi-core tape-like wires are arranged in rotational symmetry offers the advantage that the oxide superconductor in the oxide superconducting filaments permits its c axis to orient in various directions. This makes it possible to prevent the critical current from decreasing irrespective of the direction in which the magnetic field is applied and to increase the critical current density (Jc) because the oxide superconducting filament has an optimal size. The oxide superconductor should be a bismuth-based oxide superconductor, and preferably be one which has a composition of Bi2Sr2Ca1Cu2Ox.
The metal sheath may be formed from silver or a silver alloy, such that the ratio of the metal sheath to the oxide superconducting filaments is greater than 3 and smaller than 7. This makes it possible to increase further the critical current density (Jc).
In the case where the oxide superconducting wire has a rectangular shape, its cross section should have an aspect ratio greater than 1 and smaller than 6.
The oxide superconducting wire should be formed from an oxide superconductor (or a raw material thereof) in the form of powder having an average particle diameter smaller than 3 xcexcm, so that it is comparable to the conventional tape-like oxide superconducting wire in current flow characteristics and can be formed continuously.
According to the present invention, the oxide superconducting wire has better working accuracy compared with the conventional tape-like oxide superconducting wire. When the oxide superconducting wire is made into a solenoid, the resulting solenoid has a smaller deviation (in the axial and circumferential direction) than the pancake coil formed from the conventional tape-like oxide superconducting wire. Therefore, such a solenoid coil can produce a strong, uniform magnetic field.